1. Field of the Invention
The invention relates to a motor that uses attraction force generated by a permanent magnet, and an electric power supply control apparatus for the motor.
2. Description of the Related Art
In an electric power steering system that steers the steering wheels of a vehicle, for example, a surface mounted permanent-magnet motor (SPM motor) is used as the drive unit. In recent years, the demands for more efficient and higher-power electric power steering systems have been growing. However, if the motor produces higher power, motor loss torque, cogging torque, and motor inertia (system inertia) increase in proportion with the increase in the power produced by the motor. As a result, the steering feel felt by a driver deteriorates.
Although fluctuations in motor torque affect the steering feel, such torque fluctuations may be controlled by controlling the current supplied to the motor in a feedback manner. However, motor loss torque, which occurs when electric power is not supplied to the motor, may cause the deterioration of the steering feel, for example, the driver's steering wheel to turn in an unintentional direction, because feed-back control cannot be performed when electric power is not supplied to the motor. Cogging torque is also caused when electric power is not supplied to the motor. Accordingly, the cogging torque also causes deterioration of the steering feel, because feed-back control cannot be performed when electric power is not supplied to the motor. In addition, such motor loss torque and cogging torque increase in proportion to the output from the motor. This hampers increases in the motor efficiency and power output from the motor.
In addition, a high-powered motor requires a large number of magnetic circuits in a rotor and a stator. This increases the volume and rotor inertia moment. Accordingly, in the electric power steering system, compensation control is performed to reduce torque fluctuations due to such inertia. However, it is not possible to perform complete compensation due to a time constant of the motor or performance of the sensor. In this case, when the inertia is small, compensation for the fluctuation is sufficient to keep the driver from feeling the toque fluctuations. However, if the inertia is large, the inertia will affect the steering feel.
Japanese Patent Application Publication No. 2004-236369 (JP-A-2004-236369) describes a switched reluctance motor (SR motor) that produces high power and in which cogging torque is reduced. In the motor described in JP-A-2004-236369, multiple salient-pole portions, around which coils are wound and which extend inward, are arranged, along the circumferential direction, on the inner face of a stator that surrounds a rotor, and permanent magnets are buried in the salient-pole portions around which the coils are wound. When electric power is supplied to the coils, the rotor is attracted by strong magnetic attraction force due to magnetic flux synthesized from the magnetic flux generated by the coils and the magnetic flux generated by the permanent magnets. Thus, high torque is obtained. To reduce cogging torque, the following structure is employed. In the structure, the permanent magnet is buried in the salient-pole portion at a position closer to the side thereof that faces against the direction in which the rotor rotates, and the magnetic pole face of the permanent magnet faces the interior of the salient-pole portion.
However, even with the structure described in JP-A-2004-236369, transverse magnetic flux between the stator and the rotor remains due to the permanent magnet arranged at the salient-pole portion of the rotor. Accordingly, the cogging torque is not appropriately reduced. As a result, even if such a motor is used in an electric power steering system, the steering feel does not improve.